Sump pumps operate when water triggers the mechanism that activates the pump.
Tether floats hang freely from the side of the pump. As the water raises so does the float which triggers a switch that activates the pump. When the water level drops the float lowers to the side of the pump.
Vertical floats have a ball that moves up and down on a vertical rod. As the water surrounding the pump raises, the float also raises and triggers the switch and hence activates the pump. When the water level lowers the ball lowers.
A diaphragm is a membrane, located on a drum shaped mechanism on the side of the pump, which is sensitive to water pressure. As the water level raises, the water pressure increases and the diaphragm becomes concave, thereby activating the switch to turn on the sump pump. When the water level drops, the switch turns off.
A probe is sensitive to water pressure and the pump is activated electronically.

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